1. Field of the Invention
The present invention relates to a method and apparatus for calculating an exposure condition, such as at least one of a focus amount and an exposure dose, for an exposure apparatus.
2. Description of the Related Art
A conventional projection exposure apparatus projects a reticle (mask) pattern onto a substrate in manufacturing a device using the photolithography process. The recent fine patterning requires highly precise control over a focus amount and an exposure dose to realize a given resist pattern shape.
A conditioning process sets an exposure condition before a wafer used for a mass production is actually exposed. The conditioning process measures and stores in a focus exposure matrix (“FEM”) table a resist pattern for each shot on a wafer exposed by changing a focus amount (which is a wafer's position in the optical axis direction of the projection optical system in the exposure apparatus) and an exposure dose for each shot on the wafer. The conditioning process can set an exposure condition with a large margin, and expose a wafer in a mass production stage under the set exposure condition.
However, a given resist pattern may not be obtained under the condition set by the conditioning process due to various process variations. The process variations contain a resist's photosensitive sensitivity change, a thickness variation of an antireflection film under the resist, and drifts of various sensors in the exposure apparatus. Accordingly, a recently proposed measurement apparatus, such as WO2002-029870 recognizes a variation, and specifies a cause by controlling the defocus amount and the exposure dose of the exposure apparatus. This measurement apparatus photographs, through an image pickup device, a mark that is formed by exposing and developing a wafer, and detects the exposure dose or the focus amount when the mark is formed.
The conventional measuring apparatus uses two types of special grating marks shown in FIG. 9A, and measures their intervals CD1 and CD2. According to the measurement apparatus, each mark interval becomes minimum at the best focus position, and the focus amount is estimated using a characteristic (i.e., a graph shown in FIG. 9B) in which a critical dimension (“CD”) varies in an even function manner relative to a defocus amount. As understood from the graph shown in FIG. 9B, the measurement apparatus can estimate an absolute defocus value but cannot estimate its shift direction. In addition, use of a special mark complicates a detection and increases the cost.